This invention relates to a system for feedback control of the air/fuel ratio in an internal combustion engine. The system includes an air/fuel ratio detector having an oxygen-sensitive element of the oxygen concentration cell type operated with a DC current to establish a reference oxygen partial pressure in the element and provided with an electric heater to ensure proper functioning of this element. More particularly, the present invention relates to a sub-system for controlling the intensity of the aforementioned current in dependence upon the temperature of the oxygen-sensitive element with a view to keeping the element active even when not sufficiently heated.
It has become popular to control the air/fuel mixture ratio supplied to internal combustion engines to precisely a predetermined optimal value by performing feedback control with the dual objects of improving the efficiency of the engine and reducing the emission of noxious or harmful substances contained in exhaust gases.
For example, in an automotive engine system including a catalytic converter in the exhaust passage, which contains a so-called three-way catalyst that can catalyze both the reduction of nitrogen oxides and oxidation of carbon monoxide and unburned hydrocarbons, it is desirable to control the air/fuel mixture ratio to a stoichiometric ratio because this catalyst exhibits its highest conversion efficiencies in an exhaust gas produced by combustion of a stoichiometric air-fuel mixture, and also because the employment of a stoichiometric mixing ratio is favorable for high mechanical and thermal efficiencies. It is already known to perform feedback control of the air/fuel ratio in an engine system by using a sort of oxygen sensor, installed in the exhaust passage upstream of the catalytic converter, as a device for providing an electrical feedback signal indicative of the air/fuel ratio of an air-fuel mixture actually supplied to the engine. Based on this feedback signal, a control circuit commands a fuel-supplying apparatus, such as electronically controlled fuel injection valves, to control the rate of fuel feed to the engine so as to nullify or minimize deviations of actual air/fuel ratio from the intended stoichiometric ratio.
Usually the above mentioned oxygen sensor is of an oxygen concentration cell type utilizing an oxygen ion conductive solid electrolyte, such as zirconia stabilized with calcia, and conventionally the sensor is constituted of a solid electrolyte layer in the shape of a tube closed at one end, a measurement electrode layer porously formed on the outer side of the solid electrolyte tube and a reference electrode layer formed on the inner side of the tube. When there is a difference in oxygen partial pressure between the reference electrode side and measurement electrode side of the solid electrolyte tube, this sensor generates an electromotive force between the two electrode layers. As an air/fuel ratio detector for the above mentioned purpose, the measurement electrode is exposed to an engine exhaust gas while the reference electrode on the inside is exposed to atmospheric air utilized as the source of a reference oxygen partial pressure. In this state the magnitude of the electromotive force generated by this sensor exhibits a great and sharp change between a maximally high level and a very low level each time when the air/fuel ratio of a mixture supplied to the engine changes across the stoichiometric ratio. Accordingly it is possible to produce a fuel feed rate control signal based on the result of a comparison of the output of the oxygen sensor with a reference voltage which has been set at the middle of the high and low levels of the sensor output.
However, this type of oxygen sensor has disadvantages such as the significant temperature dependence of its output characteristics, the necessity of using a reference gas such as air, the difficulty in reducing its size and the insufficiency of mechanical strength.
To eliminate such disadvantages of the conventional oxygen sensor, U.S. Pat. No. 4,207,159 discloses an advanced device comprising an oxygen-sensitive element in which an oxygen concentration cell is constituted of a flat and microscopically porous layer of solid electrolyte, a measurement electrode layer porously formed on one side of the solid electrolyte layer and a reference electrode layer formed on the other side on a base plate or substrate such that the reference electrode layer is sandwiched between the substrate and the solid electrolyte layer and macroscopically shielded from the environmental atmosphere. Each of the three layers on the substrate can be formed as a thin, film-like layer. This device does not use any reference gas. Instead, a DC power supply means is connected to the oxygen-sensitive element so as to force a constant DC current (e.g. of a current intensity of about 20 .mu.A) to flow through the solid electrolyte layer between the two electrode layers to thereby cause migration of oxygen ions through the solid electrolyte layer in a selected direction and, as a consequence, establish a reference oxygen partial pressure at the interface between the solid electrolyte layer and the reference electrode layer, while the measurement electrode layer is made to contact an engine exhaust gas. Where the current is forced to flow through the solid electrolyte layer from the reference electrode layer toward the measurement electrode layer, there occur ionization of oxygen contained in the exhaust gas at the measurement electrode and migration of negatively charged oxygen ions through the solid electrolyte layer toward the reference electrode. The rate of supply of oxygen in the form of ions to the reference electrode is primarily determined by the intensity of the current. The oxygen ions arriving at the reference electrode layer are deprived of electrons and turn into oxygen molecules which results in an accumulation of gaseous oxygen on the reference electrode side of the concentration cell. However, a portion of the accumulated oxygen molecules diffuse outwardly through the microscopical gas passages in the solid electrolyte layer. Therefore, it is possible to maintain a constant and relatively high oxygen partial pressure which can serve as a reference oxygen partial pressure at the interface between the reference electrode layer and the solid electrolyte layer by the employment of an appropriate current intensity with due consideration for the microscopical structure and activity of the solid electrolyte layer. Between the reference and measurement electrode layers of this oxygen-sensitive element is generated an electromotive force, the magnitude of which is related to the composition of the exhaust gas and the air/fuel ratio of a mixture from which the exhaust gas is produced. In addition it is possible to operate this oxygen-sensitive element by forcing a current to flow therein from the measurement electrode layer toward the reference electrode layer. In this case a constant and relatively low oxygen partial pressure can be maintained at the interface between the reference electrode layer and the solid electrolyte layer.
To supply a DC current of an accurately constant intensity, use is made of a constant current supply circuit including conventional electronic control means.
The device according to U.S. Pat. No. 4,207,159 has advantages over concentration cell oxygen sensor elements in that it does not require the use of any reference gas, it can be produced in a relatively small size and exhibits good resistance to mechanical shocks and vibrations.
In practical applications it becomes necessary to provide this device (also conventional oxygen sensors of the solid electrolyte concentration cell type) with an electric heater because the activity of the solid electrolyte layer in the device becomes unsatisfactorily low when the temperature of the oxygen-sensitive element is relatively low, e.g. is below about 400.degree. C., so that the oxygen-sensitive element installed in an engine exhaust system becomes ineffective as an air/fuel ratio detector when the engine discharges a relatively low temperature exhaust gas and if the element should be heated solely by the heat of the exhaust gas. Therefore, an electric heater is usually attached to, or embedded in, the sustrate of the oxygen-sensitive element.
During the operation of this oxygen-sensitive device with the maintenance of a constant DC current flowing through the solid electrolyte layer (which has a considerable electrical resistance) an output voltage can be measured between the reference and measurement electrode layers. This voltage represents the sum of an electromotive force generated by the function of the oxygen-sensitive element as an oxygen concentration cell and a voltage developed across the reistant solid electrolyte layer by the flow of the constant current therethrough. The resistance of the solid electrolyte layer depends significantly on the temperature of this layer or the oxygen-sensitive element and greatly increases as the temperature lowers.
In an air/fuel ratio control system utilizing this oxygen-sensitive device as an air/fuel ratio detector, the value of a reference voltage, with which the output voltage of the detector is compared as an initial step in the process of producing an air/fuel ratio control signal, is determined on the assumption that the detector is sufficiently heated by the heat of the exhaust gas and by the action of the heater so that the internal resistance of the detector (principally the resistance of the solid electrolyte layer) is at a fairly low level. Usually this reference voltage is so determined as to correspond to an intended air/fuel ratio such as a stoichiometric air/fuel ratio. Where the aforementioned assumption is accurate, a basic or so-called DC level of the output voltage of the detector, excluding a variable component attributable to the electromotive force whose magnitude depends upon the composition of the exhaust gas, is not greatly different from the reference voltage. When the feedback control of air/fuel ratio is performed under such conditions actual air/fuel ratio exhibits periodic fluctuations of a certain amplitude with the target value of the control as the middle line. Therefore, the output voltage of the detector also exhibits periodic fluctuations across the reference voltage at a relatively low frequency such as several hertz. Accordingly, it is possible to continue the feedback control by appropriately altering the meaning of the air/fuel ratio control signal based upon the high-low relationship between the detector output voltage and the reference voltage so as to minmize the amplitude of the fluctuations of the actual air/fuel ratio.
When, however, the air/fuel ratio detector is operated while its oxygen-sensitive part is not sufficiently heated and hence is very high in its internal resistance, the DC level of the output voltage becomes very high and far above the determined reference voltage so that the output voltage remains above the reference voltage irrespective of the magnitude of electromotive force the element generates. Under this condition, therefore, it is impossible to perform feedback control of air/fuel ratio by utilizing the output of the detector as a feedback signal.
In practice, this situation is encountered at cold-starting of the engine. The heater in the detector is energized synchronously with ignition of the engine, and the oxygen-sensitive part of the detector is soon exposed to exhaust gas. However, the heating effects of the two heat sources are not instantaneous. The temperature of the oxygen-sensitive part rises gradually as the heater is kept working and the exhaust gas temperature rises gradually, so that the internal resistance of the oxygen-sensitive element and hence the DC level of the output voltage lowers gradually. It will be a few minutes, a relatively long period of time from the viewpoint of an electronic control technique, before the DC level of the output voltage becomes low enough to allow the output voltage to serve as a feedback signal, which becomes either higher or lower than the reference voltage depending on the direction of a deviation of the actual air/fuel ratio from the predetermined optimum air/fuel ratio, whereby feedback control of air/fuel ratio becomes practicable. For this reason, it is usual to suspend the feedback control of air/fuel ratio and to perform an open-loop control to feed the engine with a somewhat fuel-enriched mixture during the aforementioned time period. However, this is detrimental in terms of purification of the exhaust gas and improvement of fuel economy. A similar situation is encountered during idling of the engine.